Typical defects of metal lids are, for example, scratches or dents.
Examples of apparatuses for optical inspection of objects are described in patent documents EP729572B1, JP2007-024510, JP11295047A and US2008/212318.
In these solutions, the object to be inspected is illuminated in successive stages, in temporal sequence. Those solutions also involve illuminating the object with light at different frequencies to facilitate inspection of objects of different colours.
Such systems are complex, however, and have the disadvantage of requiring very long times to inspect the objects. This has a penalizing effect especially if the inspection apparatus is used in the context of a high-capacity production line. Moreover, these solutions cannot effectively distinguish between different types of defects, in particular those typical of metal lids.
Other examples of apparatuses for optical inspection of objects are described in patent documents US2008212318A1, JP2011-295047 and WO2011055397A1 (the latter in the name of the same Applicant as this invention).
In these solutions, the object to be inspected is illuminated by illuminating elements having different optical paths and irradiating light at different frequencies. This allows capturing an object in a single image containing illumination contributions of different kinds (for example, grazing light i.e. grazing light or light from above i.e. top-down light), with the possibility of separating these illumination contributions in a subsequent step of processing the image.
These solutions therefore have the advantage of guaranteeing a relatively high speed of inspection.
These solutions, too, however, are limited in their ability to reliably and effectively identify the type of defects detected, especially those typical of metal lids.